1. Field of the Invention
The present invention relates to electromagnetic wave filters employing thin film ferrite resonators and more particularly pertains to filters employing resonators having an essentially fixed resonant frequency.
2. Description of the Prior Art
At the lower microwave frequencies, cavity resonators compatible with conventional waveguides are well known for their fixed frequency operation. However, as the desired frequency of operation approaches the higher frequencies of the microwave band and still higher, the millimeter wavelength band, viz., from 24 to 300 GHz, cavity resonators are required to be very small. This small size makes the machining and fabrication of cavity resonators difficult and costly. These same considerations apply to the application of conventional waveguides to operation at the higher frequencies.
Microstrip and strip-line transmission lines and other components, including resonators, fabricated on dielectric substrates using photolithographic techniques have been used successfully at the higher microwave frequencies. However, these components tend to be relatively lossy. Therefore, resonators of this type have a relatively lower Q and, consequently, broader bandwidth then is frequently desired.
Ferrites have found broad application in microwave technology. Where a body of a ferrite material is employed as a resonator, the electromagnetic energy usually is coupled through the body by uniform spin precession about the magnetic field intensity vector in the ferrimagnetic material. The resonant frequency of such a resonator is the natural precession frequency of a magnetic dipole in the material subjected to a constant magnetic field. The natural precession frequency is a direct function of the intensity of the applied magnetic field, and of the magnetic anisotropy and saturation magnetization of the material.
Relatively high Q, narrow bandwidth resonators have been fabricated using spheres or slabs cut from single crystals of a ferrite such as, for example, yttrium iron garnet (YIG). Crystalline YIG has a cubic lattice structure. YIG resonators have been operated in their uniform spin precession resonance mode at the lower microwave frequencies. Establishing a resonance in this same mode at higher frequencies, however, requires higher applied magnetic field intensities than can be conveniently provided.
Although very high applied magnetic field intensities can be established with electromagnets having supercooled coils, the use of such electromagnets is not regarded as practical for most applications.
Resonators which depend upon the intensity of an applied magnetic field to establish their frequency of resonance, as do these prior-art resonators, suffer from an additional handicap. The resonant frequency will change or drift as the intensity of the applied magnetic field changes due to such factors as, for example, temperature variations.
Thin films of monocrystalline YIG have been deposited on single crystal substrates of gadolinium gallium garnet (GGG) and operated, in their uniform spin precession mode of resonance, as phase shifters or delay lines for magnetostatic surface waves. Examples of such surface wave devices are described in U.S. Pat. No. 3,864,647 issued Feb. 4, 1975, for "Substantially Linear Magnetic Dispersive Delay Line and Method of Operating It" granted to Bongianni, the inventor herein, and in U.S. Pat. No. 4,028,639 issued June 7, 1977, for "Oscillator Using Magnetostatic Surface Wave Delay Line," granted to Hagon et al. Such devices are subject to the same limitations as bulk YIG devices on higher frequency operation and drift due to temperature variations.
Some researchers in the field have long desired to use single crystal hexagonal ferrites in filters and related devices operating at the higher microwave frequencies and the even higher millimeter wavelength frequencies. One reason for this desire has been that single crystal material has relatively low loss. In addition, the hexagonal ferrites have a relatively very high magnetic anisotropy. Where the magnetic anisotropy is very high, the uniform spin precession mode of resonance may have a resonant frequency which is also very high. This is due to the direct dependence of this resonant frequency on magnetic anisotropy as mentioned above. Furthermore, since hexagonal ferrites such as, for example, BaFe.sub.12 O.sub.19 are known to be permanent magnet materials, it is apparent that resonators formed from them, once magnetized, will require little or no externally applied bias magnetic field to magnetically saturate the material and establish a resonant frequency for a uniform spin precession mode of resonance.
However, resonators are not easily formed from bulk single crystals of the hexagonal ferrites. In addition to their high magnetic anisotropy, these materials have a very high structural anisotropy, or anisotropy of hardness, which results in their being relatively much softer structurally along certain planes than along others. This property makes the hexagonal ferrites extremely difficult to shape and machine.
The interest in hexagonal ferrites has focused, therefore, on obtaining high quality monocrystalline films of hexagonal ferrites epitaxially grown on nonmagnetic single crystal substrates of insulator, or dielectric, material. Some early work in this field is described in U.S. Pat. No. 3,486,937 granted to Linares, Dec. 30, 1969, for "Method of Growing a Single Crystal Film of a Ferrimagnetic Material" and in Stearns et al, Materials Research Bulletin, Vol. 10, pp. 1255-1258, 1975, Pergamon Press, Inc. Later work in this field is reported in Stearns et al, Materials Research Bulletin, Vol. 11, pp 1319-1326, 1976, Pergamon Press, Inc., "Liquid Phase Epitaxy of Hexagonal Ferrites and Spinel Ferrites on NonMagnetic Spinel Substrates" and in Glass et al, U.S. patent application Ser. No. 812,862 for "Epitaxial Growth of M-Type Hexagonal Ferrite Films on Spinel Substrates and Composite" filed July 5, 1977, and assigned to the assignee of the present application. The material in the latter two references describes the manner in which thin epitaxial films of hexagonal ferrite material were prepared for the use of the inventor herein in devices conforming to the subject invention as described hereinafter.
3. Disclosure
The following references are regarded as having pertinence to this invention.
(1) Bongianni "Advanced Epitaxial Ferrite Devices," Project No. IT 161102BH57-03, Final Report, U.S. Army Research Office, Contract #DAAG29-76-C-0017, Jan. 19, 1977. PA1 (2) Lax & Button, "Microwave Ferrites and Ferrimagnetics," McGraw-Hill Book Co., Inc., New York, 1962. PA1 (3) Baynham et al, U.S. Pat. No. 3,748,605, "Tunable Microwave Filters," July 24, 1973.
(a) Sec. 7-3, "Metal-backed Slab," pp. 311-312. PA2 (b) Sec. 12-10, "Nonreciprocal Field-displacement Devices," pp. 630-637.
The contract report, written by the inventor herein, contains the first descriptions of any aspects of the present invention ever written and, starting on p. 8 thereof and in FIGS. 3, 4 and 6 thereof, presents data derived from measurements made on filters constructed in accordance with the present invention. This contract report is hereby incorporated by reference into this specification.
The first passage cited in the text by Lax & Button describes extra absorption peaks occurring in metal-backed insulating ferrite slabs. These extra absorption peaks, called "body resonances," are associated with multiple internal reflections which lead to standing waves within the slab. The condition for standing waves is given by the optical interference formula t=(2n+1).lambda./4 where n is any integer including zero, t is the slab thickness, and .lambda. is the wavelength within the ferrite.
The second passage cited in the text by Lax & Button, on pp. 636 and 637 thereof, describes the use of the oriented-magnetoplumbite (hexagonal ferrite) permanent magnet material BaFe.sub.12 O.sub.19 in a field displacement isolator operating at millimeter wavelength frequencies. Due to the very high magnetic anisotropy of the material, the isolator operates without need for an externally applied magnetic bias field.
The patent to Baynham et al, discloses a tunable, microwave frequency filter relying on the effects produced by multiple reflections of electromagnetic waves in magnetic layers sandwiched between waveguide irises, or other microwave discontinuities which are capable of producing large reflection coefficients for electromagnetic radiation.